Fuel-injected internal combustion engines fueled by liquid fuels, such as gasoline, diesel, and by alcohols, in part or in whole, such as ethanol, methanol, and the like, are well known. Internal combustion engines typically produce power by controllably combusting a compressed fuel/air mixture in a combustion cylinder. For spark-ignited engines, both fuel and air first enter the cylinder where an ignition source, such as a spark plug, ignites the fuel/air charge, typically just before the piston in the cylinder reaches top-dead-center of its compression stroke. In a spark-ignited engine fueled by gasoline, ignition of the fuel/air charge readily occurs except at extremely low temperatures because of the relatively low flash point of gasoline. (The term “flash point” of a fuel is defined herein as the lowest temperature at which the fuel can form an ignitable mixture in air). However, in a spark-ignited engine fueled by alcohols such as ethanol, or mixtures of ethanol and gasoline having a much higher flash point, ignition of the fuel/air charge may not occur at all under cooler climate conditions. For example, ethanol has a flashpoint of about 12.8° C. Thus, starting a spark-ignited engine fueled by ethanol can be difficult or impossible under cold ambient temperature conditions experienced seasonally in many parts of the world. The problem is further exacerbated by the presence of water in such mixtures, as ethanol typically distills as a 95/5% ethanol/water azeotrope.
In order to enhance the cold starting capabilities of such spark-ignited engines fueled by ethanol or other blends of alcohol, it has been proposed to provide a fuel injector of the engine with a heating element which is used to elevate the temperature of the fuel that passes through the fuel injector in route to a combustion chamber of the engine where the fuel is ignited. One heating element arrangement that has been proposed is a thick-film heater that is applied directly to the outside surface of a fuel injector body of the fuel injector. The thick-film heater may be applied to the outside surface of the fuel injector body, for example, by applying an insulating dielectric layer to the outside surface of the fuel injector body, applying two electrically conductive terminals to the insulating dielectric layer, then applying a conductive resistance top layer over the insulating dielectric layer and the two terminals. When electrical power is applied to the two terminals, current flows through the conductive resistance top layer which heats up. The generated heat passes through the fuel injector body and heats the fuel that is located within the fuel injector body. However, the thick-film heater must be controlled in order prevent over-heating. The thick-film heater may be controlled by an engine control module or a stand-alone controller, for example, by open-loop or closed-loop methods. While this thick-film heater arrangement may be effective, the need to control the think film heater may add cost and complexity to the system.
Another heating element arrangement that has been proposed is a positive temperature coefficient (PTC) ceramic heating element that is positioned around the fuel injector body of the fuel injector. When electric power is applied to the PTC ceramic heating element it elevates in temperature and the resistance of the PTC ceramic heating element increases exponentially when its temperature exceeds a threshold temperature TREF. This increase in resistance reduces the electric current that is allowed to pass through the PTC ceramic heating element, thereby allowing the PTC ceramic heating element to cool below TREF which allows the current to increase and again raise the temperature of the PTC ceramic heating element. This process repeats itself as long as the electric power is applied to the PTC ceramic heating element. In this way, the temperature of the PTC ceramic heating element is self-regulating, for example to a temperature range of about ±5° C. and the cost and complexity of controlling the temperature used in the previously described thick-film heater arrangement is avoided. The self-regulating temperature occurs at the Curie temperature of the PTC ceramic heating element. The Curie temperature of the PTC ceramic heating element is the temperature at which a phase change in the structure occurs, thereby changing from more crystalline structure to a more amorphous structure. This change in phase is responsible for the increase in electrical resistance of the PTC ceramic heating element and is characterized by significant mechanical dimension changes measured as the coefficient of thermal expansion (CTE). The CTE of the PTC ceramic heating element is typically greatest above the Curie temperature.
Japanese patent application publication number JP 2003-13822A describes a fuel injector with one arrangement for a ceramic heating element which is formed as a hollow cylinder and press fit closely over the metal fuel injector body. The close press fit of the cylindrical ceramic heating element over the fuel injector body mechanically stresses the ceramic heating element when the metal body that it surrounds expands preferentially with rising temperature, which may cause the ceramic heating element to crack. Providing a sufficiently wide annular clearance between the ceramic element and the fuel injector body that it surrounds to accommodate the differential thermal expansion severely reduces the thermal conductivity, as does any dead air space. Adding known thermally conductive materials in the annular space, such as solder or conductive adhesives, improves conductivity, but effectively reintroduces the effect of a close press-fit.
U.S. Pat. No. 6,578,775 to Hakao describes a fuel injector with another arrangement for a ceramic heating element, obviously a response to the problems outlined above. Hakao describes a pair of arc-shaped ceramic heating elements that are pressed onto the outer periphery of the fuel injector body by a resilient clip or heater holder. By, in effect, pre-breaking the cylindrical ceramic piece into a pair of arc-shaped ceramic heating elements, the risk of cracking the ceramic heating elements present in JP 2003-13822A as described earlier is mitigated. However, the effectiveness of the ceramic heater arrangement of Hakao is reduced because the entire perimeter of the fuel injector body is not heated and the complexity of the heating arrangement is increased by the additional electrical terminals that are needed in order to apply electric power to each ceramic heating element, as well as the resilient press-fit mechanism.
What is needed is a heated fuel injector which minimizes or eliminates one or more of the shortcomings as set forth above.